The use of electrical measurements in prior art downhole applications, such as logging while drilling (LWD) and wireline logging applications is well known. Such techniques may be utilized to determine a subterranean formation resistivity, which, along with formation porosity measurements, is often used to indicate the presence of hydrocarbons in the formation. For example, it is known in the art that porous formations having a high electrical resistivity often contain hydrocarbons, such as crude oil, while porous formations having a low electrical resistivity are often water saturated. It will be appreciated that the terms resistivity and conductivity are often used interchangeably in the art. Those of ordinary skill in the art will readily recognize that these quantities are reciprocals and that one may be converted to the other via simple mathematical calculations. Mention of one or the other herein is for convenience of description, and is not intended in a limiting sense.
Formation resistivity (or conductivity) is commonly measured by transmitting an electromagnetic wave through a formation using a length of antenna wire wound about a downhole tool. As is well known to those of ordinary skill in the art, a time varying electric current (an alternating current) in a transmitting antenna produces a corresponding time varying magnetic field in the formation. The magnetic field in turn induces electrical currents (eddy currents) in a conductive formation. These eddy currents further produce secondary magnetic fields which may produce a voltage response in a receiving antenna. The measured voltage in the receiving antennae can be processed, as is known to those of ordinary skill in the art, to obtain one or more measurements of the secondary magnetic field, which may in turn be further processed to estimate formation resistivity (conductivity) and/or dielectric constant. These electrical formation properties can be further related to the hydrocarbon bearing potential of the formation via techniques known to those of skill in the art.
It is also well known that a transmitted electromagnetic wave is typically both attenuated and phase shifted by an amount related to the resistivity and/or dielectric constant of the formation. The transmitted wave is commonly received at first and second spaced receiving antennae. The attenuation and phase shift between the first and second receivers may be acquired by taking a ratio of the received waves. The attenuation and/or phase shift may then be utilized to estimate the formation resistivity and/or dielectric constant. In order to acquire more data, e.g., at multiple depths of investigation into the formation, it is well known to make the above measurements using multiple spaced transmitters since the depth of penetration of an electromagnetic wave into the formation tends to increase with increased spacing between the transmitter and receiver. The use of multiple perturbation frequencies is also a known means of investigating multiple depths of investigation since the depth of penetration tends to be inversely related to the frequency of the propagated electromagnetic waves.
FIG. 1 depicts a well known and commercially available prior art resistivity tool 50. The tool embodiment depicted includes first and second receivers R deployed symmetrically between multiple pairs of transmitters T. In use the transmitters are typically fired sequentially and the corresponding induced voltage response measured at each of the receivers. The attenuation and phase difference may be determined by taking a ratio of the received voltage signals. To compensate for instrument noise and/or other borehole effects the attenuation and phase difference from each of the transmitter pairs may be averaged to essentially cancel out the error term.
Directional resistivity measurements are also commonly utilized to provide information about remote geological features (e.g., remote beds, bed boundaries, faults, and/or fluid contacts) not intercepted by the measurement tool. Such information includes, for example, the distance from and direction to the remote feature. In geosteering applications, directional resistivity measurements may be utilized in making steering decisions for subsequent drilling of the borehole. For example, an essentially horizontal section of a borehole may be routed through a thin oil bearing layer. Due to the dips and faults that may occur in the various layers that make up the strata, the distance between a bed boundary and the drill bit may be subject to change during drilling. Real-time distance and direction measurements may enable the operator to adjust the drilling course so as to maintain the bit at some predetermined distance from the boundary layer. Directional resistivity measurements also enable valuable geological information to be estimated, for example, including the dip and strike angles of the boundary as well as the vertical and horizontal conductivities of the formation.
Methods are known in the art for making LWD directional resistivity measurements and commonly involve transmitting and/or receiving transverse (x-mode or y-mode) or mixed mode (e.g., mixed x- and z-mode) electromagnetic waves. Various tool configurations are known in the art for making such measurements. In one advantageous tool configuration, a plurality of collocated x-mode and z-mode transmitting and receiving antennae are utilized. A tool configuration similar to that depicted on FIG. 1 may be utilized with each of the transmitters and receivers including such a collocated antenna arrangement.
One drawback with such an arrangement is that it requires a large number of sequential transmitter firings within each measurement cycle. This drawback is particularly acute for directional resistivity tools since a directional resistivity tool must make, for a single depth level, multiple measurements corresponding to various azimuth angles to provide sufficient azimuthal coverage. A large number of transmitters combined with a large number of azimuthal measurements can lead to an enormous number of sequential firings. For example, the tool configuration depicted on FIG. 1 requires 12 sequential firings per measurement cycle (x-mode and z-mode firings at each transmitter) in which the x-mode transmitter must be fired for a sufficient length of time period to allow collection of multiple measurements at various azimuth angles. Such a large number of transmitter firings reduces the allowable acquisition time for each firing and therefore tends to reduce data accuracy. Therefore, there remains a need in the art for an improved directional resistivity tool and in particular a tool configuration that reduces the number of required transmitter firings.